In wireless communication, offset between the crystal oscillator frequencies of a transmitting end and a receiving end, or some other factors (such as different systems, circuits or working environments), results in the carrier frequency of a transmitted signal being not identical with the local oscillator frequency at the receiving end, thus causing carrier frequency offset between the transmitting end and the receiving end. Moreover, the offset frequency between the two ends may also vary with time and the external environment, forming a phenomenon of carrier frequency offset drift. And the carrier frequency offset is also common in wireless communication.
In a currently employed wireless receiving system, a low-pass filter is provided at a receiver to filter out background noise and out of band interference, so as to ensure reception performance of the entire receiver. The low-pass filter is designed such that the transition band thereof is as short as possible and rejection capability of the stopband is as great as possible, for a better filtering effect of noise and interference. And usually, the pass-band width of the low-pass filter is set as a fixed value close to the width of an effective signal.
In the presence of the carrier frequency offset, however, providing a narrow fixed low-pass filter may directly filter out part or all of the useful signals, and even in the case of using feedback compensation, an error would occur in frequency offset estimation, provided that the noise is relatively great or excessive energy is filtered off the effective signal, thus ultimately affecting the entire demodulation performance. And in the case of enlarging the pass-band width of the low-pass filter (e.g. to the sum of the signal bandwidth and a possible maximum carrier frequency offset), greater noise would be introduced, thereby causing a further decline in the reception performance, and particularly, when the carrier frequency offset frequency and the signal bandwidth are comparable with each other, an increase in the bandwidth of the filter would have a significant impact on the performance.